PROJECT SUMMARY Heart failure is a leading cause of mortality throughout the world. The only viable life-sustaining option for patients with advanced heart failure is a heart transplant. Due to the complex nature of the heart transplant procedure and the limited supply of donor hearts, most of these patients tend to use Left Ventricular Assist Devices (LVADs) either to bridge the gap until they can undergo heart transplant or as the permanent lifetime option. When the patient leaves their home, function of the LVAD totally and critically relies on the battery power. Currently LVADs are powered by lithium (Li)-ion batteries, which are bulky, heavy (~2 lb), and can power LVADs only for ~10-12 hours. If the patient is more active or emotionally stressed, batteries will power the system for a much shorter period of time. Frequent replacing the depleted batteries with charged batteries becomes more challenging for elderly patients and patients with limited mobility. Furthermore, this could create an emergency situation if the patient with the LVAD is not in a position to recharge the batteries. Existing LVAD battery packs are heavy mainly due to the insufficient energy density of the Li-ion batteries. Thus, to provide the needed energy, several of these Li-ion battery cells are incorporated in to the existing LVAD battery packs to power LVAD for ~10-12 hours. This approach has significantly increased the total weight of the LVAD battery packs. The only option to decrease the weight of LVAD battery pack and increase the battery life on a single charge is to introduce a new type of battery chemistry that can provide high energy density. We envision the development of a reliable, lightweight battery pack for LVAD using the Li-sulfur chemistry because of its significantly higher theoretical specific capacity and specific energy density (potentially five times higher than that of commercialized Li-ion batteries). In this proposed study, we will develop Li-sulfur batteries (E-Light batteries) with significantly higher energy density than currently used Li-ion batteries. The result will be a battery pack with a remarkable impact on powering LVADs. Our E-Light batteries can increase the usage time of LVAD to 24 hours, and it will also reduce the weight of the battery by about 2x. In this proposed study, we will extensively address technical hurdles associated with the current Li-sulfur batteries that limit their application in medical devices. In the Phase I study, we will introduce several key innovations to develop Li- sulfur batteries that can deliver high energy density with long cycle life. We will also demonstrate the feasibility of the lab-scale Li- sulfur batteries to fabricate E-Light batteries, which will establish the potential of our E-Light batteries to power LVADs. Once E-Light battery is developed, these batteries could also power many other biomedical devices.